In recent years, there has been a growing demand for a higher recording density of a disk used as an information recording medium for computers or other electronic equipment, while the disk has been reduced to a smaller size.
A hard disk is one of the information recording media, which is formed by applying a magnetic film to a surface of a glass substrate or aluminum substrate as a base. The magnetic film should have a flat and smooth surface without concave and convex portions. Thus, the surface of the magnetic disk (hereinafter simply referred to as the disk) is polished and flattened in the varnishing process. However, protrusions may remain even after the surface has been flattened in the varnishing process. For this reason, the disk is tested by a protrusion testing device to check if there is a remaining protrusion on the surface of the disk. When a remaining protrusion is found, the disk is returned to the varnishing process and is polished again.
The protrusion testing device includes a piezoelectric sensor (piezoelectric element) as a protrusion detection sensor on the upper side of a slider of a thin film head. The protrusion testing device is designed to test the presence of a protrusion on a disk in the following way. The disk is rotated at a predetermined circumferential speed. A predetermined flying height is given to the slider (in which the thin film header may be or may not be provided). Then, the vibration generated when the slider collides with a protrusion on the disk is converted to an electrical signal by the protrusion detection sensor. In this way, a protrusion detection signal is obtained. In particular, the present applicants have already applied for a patent on a protrusion testing device for testing surface irregularity on a disk, in addition to testing protrusions as described above (JP-A No. 305286/2007).
It is to be noted that an ultrasonic sensor may be used for the protrusion detection sensor in place of the piezoelectric element.
In the currently used high density recording disk with 10000 or more tracks/inch2, the distance between the magnetic head and the disk is reduced from several tens of nm to several nm. Hence, the flatness of the disk surface has been emphasized in recent years.
When the distance between the magnetic head and the disk is reduced from several tens of nm to several nm, the height of a protrusion to be detected is also reduced to several tens of nm or less. Further, not only the protrusion of the disk but also the surface deflection and flatness of the rotating disk are a problem in the detection of such a protrusion height.
The surface deflection is caused by the irregularity of the surface of the disk, and by the degradation of the balance due to the eccentricity of the disk with respect to the axis of rotation. In particular, in the recent glass disk with a thickness from 0.3 mm to 0.5 mm, surface deflection is likely to occur due to the irregularity of the surface of the disk when it is rotated.
As described above, when the distance between the magnetic head and the disk becomes small, the surface deflection of the disk increases, or the flatness of the disk is degraded. This will cause an error in writing and reading, or lead to a head crash. As a result, even if a protrusion with a considerable height is not present on a disk, the disk is typically determined to be failed.
Thus, a measurement circuit for obtaining a signal of an average value of the track and the like is added to a protrusion detection circuit of the protrusion testing device as disclosed in JP-A No. 305286/2007, in order to measure the flatness of the disk.
The measurement circuit for obtaining average value signals can obtain signals relating to the flatness in the following way. A signal of about 100 kHz to 500 kHz is obtained by a piezoelectric sensor through air vibration generated by the disk rotation. The obtained signal shows the concave-convex state of the disk surface. Such a signal can be obtained by the piezoelectric sensor mounted on the upper portion of a slider of 5 mm square or less.
The currently used protrusion testing device measures the flatness of a disk in the following way. A protrusion detection circuit detects the peak of a signal from the piezoelectric element. An average value calculation circuit disposed parallel to the protrusion detection circuit detects the average value of the signal from the piezoelectric element, to calculate the average value for each track. The average value is converted from an analog signal to a digital signal. Then, the average value for each track is compared with a predetermined reference value in a data processing device. When the obtained average value does not exceed the predetermined reference value, it is determined that the flatness of the disk is within the specifications.
However, when such an average value calculation circuit is disposed parallel to the protrusion detection circuit, data should be processed by converting from an analog signal to a digital signal. For this reason, it is necessary to have a circuit for switching between the protrusion detection circuit and the average value calculation circuit. This leads to a problem that the disk test efficiency is reduced.
Parallel processing of data is possible by providing an A/D conversion circuit both in the protrusion detection circuit and in the average value calculation circuit. However, a D/A conversion circuit of 16 bits or more is necessary to ensure the measurement accuracy. This leads to an increase in size of the measurement circuit.